The invention relates generally to the field of optical modulation. In particular, the invention relates to methods and apparatus for high-efficiency electro-optic modulation.
Optical modulators impress or modulate RF (or microwave) electrical signals onto a light beam in order to generate a modulated optical beam that carries data. Modulators either directly modulate the optical beam as it is generated at the optical source or externally modulate the optical beam after it has been generated. Direct modulation may be accomplished by modulating the drive current of the optical source. Direct modulation may also be accomplished by modulating the optical intensity of light leaving the source with an integrated electro-absorptive modulator.
External modulation can be accomplished by using an external modulator that is separate from the optical source. External modulation is advantageous because it can modulate signals over a very wide bandwidth. External modulators are typically voltage-controlled devices that include a traveling-wave electrode structure, which is positioned in close proximity to the optical waveguide. The electrode structure produces an electric field that overlaps the optical waveguide over a predetermined distance (the interaction length) and causes an electromagnetic interaction, which modulates the optical signal.
Lithium niobate (LN) electro-optic external modulators are increasingly being used to modulate data on optical signals that are being transmitted at very high data rates and over long distances. Lithium niobate modulators are advantageous because they can modulate optical signal over a broad frequency range, they modulate optical signals with controlled, potentially zero, optical frequency shift (frequency xe2x80x9cchirpxe2x80x9d), and they operate over a broad wavelength range. These features are particularly desirable for Dense Wavelength Division Multiplexing (DWDM) broadband optical communication systems that transmit optical signals with many optical wavelengths through a single optical fiber.
Lithium niobate crystals have an inherent mismatch between the velocity of optical and electrical signals propagating through the crystal, which lowers modulation efficiency. The RF propagation index is significantly higher than the optical refractive index of lithium niobate. That is, the lithium niobate crystal slows the RF signal relative to the optical signal so that it takes the RF signal a longer period of time to travel over the interaction distance. Thus, the RF signal becomes out-of-phase with or xe2x80x9cwalks offxe2x80x9d the accumulated modulation on the optical signal.
This xe2x80x9cwalk offxe2x80x9d lowers the modulation efficiency. Modulators used for transmission at high speeds and over long distances must be efficient to minimize the use of electronic amplifiers and digital drivers. Electronic amplifiers and digital drivers are costly and occupy valuable space in the transmission link. In addition, electronic amplifiers and digital drivers may fail and lower the quality of service and require expensive maintenance in the field.
FIG. 1 illustrates a top view of a prior art electro-optic device 10 that increases modulation efficiency by compensating for the velocity mismatch between the optical and electrical signals propagating through the device by using phase reversal sections that are co-linear with the optical waveguide. The device 10 includes an optical waveguide 12 and RF electrodes 14 that are positioned in zero degree phase sections 16 and phase reversal sections 18.
The phase reversal sections 18 periodically flip the RF electrodes 14 to either side of the optical waveguide 12 to produce a 180 degree phase shift in the RF signal relative to the accumulated modulation on the optical signal. The RF electrodes 14 are positioned to alternate between the zero degree phase sections 16 and the phase reversal sections 18. The length of the zero degree phase shift sections 16 is chosen so that the RF signal xe2x80x9cwalks offxe2x80x9d the accumulated modulation on the optical signal approximately 180 degrees before it is flipped 180 degrees in the phase reversal sections 18.
The prior art electro-optic device 10 of FIG. 1 has relatively low modulation efficiency per unit length. This is because the phase of the RF signal is modified with co-linear sections that are positioned at intervals of 180 degrees. When the difference in phase between the RF and optical signals approaches 180 degrees, the incremental increase in modulation depth with incremental change in electrode length approaches zero. Therefore, the total length of the device must be significantly increased in order to achieve the required modulation.
FIG. 2 illustrates a top view of a prior art electro-optic device 30 that increases modulation efficiency by compensating for the velocity mismatch between the optical and electrical signals propagating through the device 30 by using co-linear but intermittent interaction sections. The device 30 includes an optical waveguide 32 and RF electrodes 34 that are positioned to alternate between an interaction region 36 and a non-interaction region 38 relative to the optical waveguide 32.
The length of the interaction region 36 is chosen so that the RF signal xe2x80x9cwalks offxe2x80x9d the modulation on the optical signal by as much as 180 degrees of phase shift before it is routed away from the optical waveguide 32 in a co-linear direction and into the non-interaction region 38. The length of the non-interaction region 38 is chosen so that the RF signal becomes phase matched with the accumulated modulation on the optical signal at the end of the non-interaction region 38. The prior art modulator of FIG. 2 has non-interaction regions of substantial length that re-align the phase of the modulating signal, but introduce RF loss, and occupy device length.
Increasing the length of a lithium niobate electro-optic modulator increases the size of the package. Increasing the size of the modulator package and the power supplies is highly undesirable because the space on transmitter boards and in transmission huts is very limited. Efficiency is also a major consideration, as more powerful electronic drivers require more space on the transmitter card. State-of-the-art DWDM transmission equipment occupies a significant amount of space because the equipment includes electronics for numerous channels. Most transmission huts were designed for much more modest communication systems and are not very spacious. Many transmission huts cannot be expanded for various reasons.
Another disadvantage of prior art electro-optic modulators in FIGS. 1 and 2 is that these modulators are not suitable for modulating digital signals. This is because these modulators have non-linear phase characteristics as a function of frequency response. Therefore, the digital pulse shapes are not preserved. In addition, the efficiency is concentrated in a set of narrow band regions, which is suitable for a square wave signal, but is unsuitable for digital signals having an arbitrary bit sequence. Other prior art modulators correct the uniformity of efficiency with frequency by using a periodic, Barker-code, phase reversal locations along the modulator length. However, these prior art modulators still have non-linear phase as a function of frequency.
Some prior-art electro-optic modulators uses a buffer layer to achieve velocity matching as described in connection with FIG. 3. These prior art devices have non-optimized modulation efficiency because they preserve significant modulation beyond the required bandwidth.
Some prior art electro-optic modulators use z-cut lithium niobate. Using z-cut lithium niobate is advantageous because z-cut lithium niobate inherently provides better overlap between optical and RF fields and thus, has an inherently high modulation efficiency as compared with x-cut lithium niobate electro-optic modulators. Z-cut lithium niobate electro-optic modulators, however, experience bias drift effects. Conductive buffer layers and charge bleed-off layers are typically used to mitigate these bias drift effects. Including a conductive buffer layer and charge bleed-off layer adds significantly to the fabrication cost associated with z-cut electro-optic modulators. In addition, prior art z-cut lithium niobate electro-optic modulator preserve significant modulation beyond the required bandwidth and, therefore do not have optimized efficiency.
The present invention relates to high-efficiency electro-optic modulation and to electro-optic modulators for modulating digital signals with equalized frequency response. A discovery of the present invention is that modulation efficiency of electro-optic modulators can be increased by discarding excess modulation efficiency in regions inside and outside the bandwidth of the digital spectrum. Another discovery of the present invention is that the modulation efficiency and the frequency response of an electro-optic modulator can be independently optimized by first choosing an electrode geometry that corresponds to a modulation efficiency in the digital signal spectrum. Compensation networks are then added at various points along the length of the modulating electrode in order to modify the frequency response of the modulator to discard excess modulation efficiency where it is not needed, and add efficiency where it is needed. Optimum modulation efficiency and frequency response are achieved by iterating the selection of the electrode geometry and position and type of compensation network.
Accordingly, the present invention features an electro-optic modulator for modulating a digital signal, such as a Mach Zehnder interferometric modulator, comprising an optical waveguide formed in an electro-optic material that propagates an optical signal along a first direction of propagation. A buffer layer may be formed on the optical waveguide to at least partially velocity match the electrical modulation signal to an optical signal.
An electrical waveguide is formed on the electro-optic material and positioned generally co-linear relative to the optical waveguide and in electromagnetic communication with the optical waveguide. The electrical waveguide propagates an electrical modulation signal in the first direction of propagation. The electrical waveguide may provide chirp modulation.
The electrical waveguide includes hot and ground electrical waveguides positioned proximate to the arms of the Mach Zehnder interferometric modulator. In one embodiment, the electrical waveguide comprises a co-planner strip electrode. A driver is coupled to the electrical waveguide. In one embodiment, the driver has a frequency response that is complementary to an electro-optic frequency response of the modulator.
The geometry of the electrical waveguide is selected to achieve a modulation efficiency at a frequency in a bandwidth of a digital spectrum. At least one of the buffer layer thickness, buffer layer dielectric constant, and electrode geometry are selected to achieve a modulation efficiency or to maximize the modulation efficiency at the frequency in the bandwidth of the digital spectrum. In one embodiment, the electrical waveguide geometry is selected to achieve the modulation efficiency at substantially a mean frequency in the bandwidth the digital spectrum.
In one embodiment, the electrical waveguides comprise dual-drive electrical waveguides including a first and second pair of hot and ground electrical waveguides positioned proximate to a first and second arm of the Mach Zehnder interferometric modulator, respectively. A driver is coupled to each pair of hot and ground electrical waveguides. In one embodiment, a frequency response of each driver is complementary to the electro-optic frequency response of the modulator.
A compensation network is electrically coupled to the electrical waveguide at a junction. The compensation network may comprise a plurality of compensation networks where a respective one of the plurality of compensation networks is electrically coupled to the electrical waveguide at a respective one of a plurality of junctions. The compensation network modifies the electro-optic response of the electro-optic modulator below a mean frequency of the digital spectrum, thereby causing a magnitude of the electro-optic response to increase in the bandwidth of the digital spectrum. The compensation network modifies at least one of a phase or an amplitude of the electrical modulation signal at the junction relative to a phase or an amplitude of the optical signal at the junction, respectively.
There are numerous embodiments of the compensation network. The compensation network may include an RF time delay network or a polarity reversal section. In one embodiment, the compensation network propagates the electrical signal in a second direction of propagation that is substantially non-co-linear with the first direction of propagation. The electrical loss per unit length of the compensation network may be lower than an electrical loss per unit length of the electrical waveguide. The compensation network may be removably attached to the electro-optic modulator. Also, the temperature dependence of the compensation network may be inversely proportional to a temperature dependence of the electro-optic material.
The present invention also features a method of independently controlling modulation efficiency and electro-optic response of an electro-optic modulator for modulating a digital signal. The method includes selecting a modulator length corresponding to a modulator bandwidth. The modulation efficiency is then adjusted at a frequency in the bandwidth of a digital spectrum by selecting an electrical waveguide geometry of the electro-optic modulator.
The magnitude of the electro-optic response is then reduced below a mean frequency of the digital spectrum. In one embodiment, the magnitude of the electro-optic response is reduced electro-optically. In some embodiments, the electro-optic response above the mean frequency of the digital spectrum is increased. The steps of choosing the modulator length, adjusting the modulation efficiency, and reducing the magnitude of the electro-optic response below the mean frequency of the digital spectrum may by iterated to achieve a predetermined modulation efficiency and electro-optic response in the bandwidth of the digital spectrum.
The present invention also features a method of independently optimizing the modulation efficiency and electro-optic response of an electro-optic modulator for modulating a digital signal. The method includes selecting a modulator length that corresponds to a modulator bandwidth. The modulation efficiency is then optimized at a frequency in the bandwidth of a digital spectrum by selecting an electrical waveguide geometry of the electro-optic modulator.
The electro-optic response is optimized above the mean frequency of the digital spectrum by reducing a magnitude of an electro-optic response below the mean frequency of the digital spectrum. The steps of selecting the modulator length, optimizing the modulation efficiency, and optimizing electro-optic response above the mean frequency of the digital spectrum may by iterated to achieve a predetermined modulation efficiency and electro-optic response in the bandwidth of the digital spectrum.